I. Field of the Invention
The present invention relates to manufacturing method of an impeller used in a centrifugal rotor such as a centrifugal compressor.
Priority is claimed on Japanese Patent Application No. 2009-015451, filed on Jan. 27, 2009, the contents of which are incorporated herein by reference.
II. Description of the Related Art
As an impeller used for a centrifugal rotor such as a centrifugal compressor, an impeller (a closed impeller) is known to which a cover is attached, which includes a disc attached to a rotation shaft, a cover opposed to the disc so as to have a space, and a plurality of blades connecting the disc with the cover. In the cover-attached impeller, the space which is enclosed by the surfaces of the plurality of blades and surfaces of the cover and the disc which are opposed to each other, functions as a passage for compressing gas. The impeller provided in the centrifugal compressor is manufactured by a method such as integral molding by casting, bonding by welding, or bonding by brazing (or transient liquid phase diffusion bonding, solid-phase diffusion bonding).
Specifically, the integral molding is a method which involves integrally forming the cover, the blades, and the disc by the machining of materials. However, in the impeller generally provided in the centrifugal compressor, as the passage has a complex shape which is curved in both the axial direction (the rotation shaft direction) and the radial direction, the integral molding is difficult.
Bonding by welding is a method which involves bonding an integral member made by forming blades integrally with either a cover or a disc to the other one of the cover and the disc by welding, or involves bonding the blades, the cover, and the disc to each other by welding. In this case, it is necessary to insert a welding torch into a passage, and in a case where the passage is narrow, inserting it into the passage is difficult. Therefore, there is a problem in that welding defects are more likely to occur.
The bonding by brazing is a method used when a cover to which blades are attached is bonded to a disc, and involves disposing a brazing filler metal such as foil, powder, or wire at the bonding points, placing the cover and the disc to overlap with each other in the furnace, and bonding them by heating (for example, refer to JP-A-2003-328989). The bonding by brazing has an advantage in that it can be easily performed compared with the bonding by welding described above even in cases where the passage is narrow.
Conventionally, an impeller, in which a portion around a rotation shaft (inner periphery) of a disc which is a bond part bonded with a blade, is formed with a curved surface in order to enhance aerodynamic performance. FIG. 7 is a side cross-sectional view illustrating the shape of an impeller having such a configuration. The impeller 10 illustrated in FIG. 7 has a passage R of which an outer peripheral side is formed along a radial direction by a disc 11 and a cover 12 and which increasingly curves toward the cover 12 in the direction toward the inner peripheral side. The disc 11 and the cover 12 are provided with curved surfaces 11a and 12a along a curved surface Ra of the passage R.
However, the structure of the existing impeller has the following problems.
In the above-mentioned impeller provided in the centrifugal compressor, tensile force acts on the bond part between the blade and the disc or the cover due to rotation in a direction in which the cover and the disc are separated from each other by centrifugal force, and bending stress occurs in a direction relative to the rotation direction. Accordingly, excess concentrated stress acts on the bond part corresponding to base portions of the blade and the disc. Particularly, in the bond part, rotation stress on the inner peripheral side in the radial direction increases.
Therefore, reliable bonding is required on the inner peripheral side of the bond part. However, in the case of the existing impeller 10 described above, in which the curved surface 11a is formed in the vicinity of the rotation shaft (inner periphery) of the disc 11 of the bond part with the blade 13 as illustrated in FIG. 7, bonding is performed in a state where the disc 11 faces down. That is, bonding is performed in a state where the outer peripheral side of the curved surface 11a of the disc 11 is lower than the inner peripheral side thereof. Accordingly, when the brazing filler metal 14 disposed between the blade 13 and the disc 11 is melted and liquefied, the brazing filler metal 14 flows from the inner peripheral side toward the outer peripheral side (in an arrow direction G) along the curved surface 11a. Therefore, the brazing filler metal 14 is insufficiently supplied to the inner peripheral side of the bond part, such that there is a problem in that the bonding strength of the bond part on the inner peripheral side, which requires a reliable bonding, is reduced.
The present invention is designed to solve the above-mentioned problems. An object of the present invention is to provide a manufacturing method of an impeller, which ensures bonding strength by preventing insufficient supply of bonding material.